1. Field of Invention
The invention relates generally to phosphor-converted semiconductor light emitting devices.
2. Description of Related Art
Light emitting devices including light emitting diodes (LEDs) are well known solid state devices that can generate light having a peak wavelength in a specific region of the light spectrum. LEDs are typically used as illuminators, indicators, and displays. LEDs based on the III-nitride materials system have been developed that can efficiently emit light in a relatively narrow band around a peak wavelength in the blue to UV range of the spectrum. Since blue-UV light has a higher photo energy relative to other colors of visible light, such light generated by III-nitride LEDs can be readily converted to produce light having a longer wavelength. It is well known in the art that light having a first peak wavelength (“primary light”) can be converted into light having a longer peak wavelength (“secondary light”) using a process known as luminescence. The luminescence process involves absorbing the primary light by a photoluminescent phosphor material, which excites the atoms of the phosphor material, and emits the secondary light. The peak wavelength and the band of wavelengths around it (in short wavelength) of the secondary light will depend on the phosphor material. The type of phosphor material can be chosen to yield secondary light having a particular peak wavelength.
U.S. Pat. No. 5,998,925, granted to Shimizu et al., teachings a “white light emitting diode comprising a light emitting component using a semiconductor as a light emitting layer and a phosphor which absorbs a part of light emitted by the light emitting component and emits light of wavelength different from that of the absorbed light, wherein the light emitting layer of the light emitting component is a nitride compound semiconductor and the phosphor contains garnet fluorescent material activated with cerium which contains at least one element selected from the group consisting of Y, Lu, Sc, La, Gd and Sm, and at least one element selected from the group consisting of Al, Ga and In and, and is subject to less deterioration of emission characteristic even when used with high luminance for a long period of time.” See U.S. Pat. No. 5,998,925, abstract.